1. Field of the Invention
The present invention relates to multi-layer gratings/mirrors and their application in analyzing systems.
2. Discussion of Related Art
It is well known in the art to use large d-spacing artificially grown crystals, e.g. PET crystals, for x-ray fluorescence spectroscopy. Such artificial crystals are very sensitive to the environment and degrade quickly under the influence of radiation, heat, moisture, etc. Accordingly, the artificial crystals do not last long and are difficult to use. In hard x-rays region (wavelength less than 0.2 nm) ordinary crystals, e.g. Si or Ge crystals, when used in monochromators and spectrometers in some cases have resolving powers that are too high (xcx9c104) and cut out too much useful flux. Accordingly, the measurement times become longer.
While reflective gratings can be used in place of the artificial crystals, such gratings suffer from low diffraction efficiency in the softer x-ray region (wavelengths ranging from about 0.2 nm to about 1.2 nm). Multilayer mirrors, while improving reflection efficiency relative to the gratings in a wide wavelengths range (from 1 nm to 20 nm), have a resolving power that is too low xcx9cxcex/xcex94xcexxcx9c10-100.
Multilayer gratings/mirrors are well known in the art and are very stable, durable and are easy in use. Examples of such multilayer gratings/mirrors are described in U.S. Pat. Nos. 4,727,000; 5,646,976 and 5,757,882, the entire contents of each are hereby incorporated herein by reference. It is well known that the bandpass of such multilayer gratings/mirrors is defined by the number of bilayers in which the incident wave penetrates. This number of bilayers in the multilayer grating/mirror is limited by the factor that due to interference in, the periodical structure of the multiple layers stacked upon one another, the radiation wave incident on the multilayer grating/mirror is reflected back and does not penetrate any deeper than a so-called extinction depth. The extinction depth is determined by the wavelength of the incident radiation and the materials of the multilayer grating/mirror.
The bandpass and correspondingly the resolution of a spectrometer or a monochromator that uses a multilayer grating/mirror is proportional to:
xcex94xcex/xcexxcx9c1/N,xe2x80x83xe2x80x83(1)
wherein N is the number of multilayer periods present within the extinction depth. In many instances, the resolution of a spectrometer or a monochromator is required to be better than that as determined by the extinction depth in the manner described above.
Based on the above relationship, one way to increase the resolution of a multilayer grating/mirror is to increase the extinction depth and thus the number N of multilayer periods within the extinction depth. One known way to increase the extinction depth is to etch grooves in the multilayer grating/mirror and remove part of the reflection planes so as to allow the incidence radiation wave to penetrate deeper into the multilayer grating. As a result, the number of layer N in the extinction depth increases and the bandpass, or the resolution, increases in accordance with equation (1) above. Such a multilayer grating/mirror is discussed in the paper entitled xe2x80x9cLamellar Multilayer Gratings with Very High Diffraction Efficiency,xe2x80x9d V.V. Martynov et al., SPIE Vol. 3150 0277-786X/97, pp. 2-8.
By changing the groove/period ratio of the multilayer grating/mirror, the amount of removed material can be continuously varied and, thus, the extinction depth and resolution can be continuously varied. The maximum practical factor in the bandpass variation is defined by technological limits and, for different wavelength, can be as from 1 to 100. While such a multilayer grating/mirror provides increased resolution, the multilayer grating/mirror with periodically spaced lands also generates many diffraction orders, which contribute in making the detector signal to have a small signal to noise ratio. The generation of multiple diffraction orders is shown by analogy to the single layer periodic transmission grating diffraction intensity distribution shown in FIG. 3. Obviously, if a single layer transmission grating with a periodic structure generates multiple diffraction orders, then a multi-layer transmission grating with a periodic structure will also generate multiple diffraction orders.
Accordingly, it is an objective of the present invention to provide a multilayer grating/mirror that for a wide range of wavelengths has increased resolution and diffraction efficiency while at the same time contributing in making the detector signal having a large signal to noise ratio.
Another object of the present invention is to provide crystals that are not sensitive to the environment and do not degrade quickly when used in an analyzing system.
One aspect of the present invention regards an analyzing system that includes a radiation generator that generates a beam of radiation along a first direction and a grating that receives the beam of radiation and generates a second beam of radiation that possesses only a zeroth order of diffraction. An object receives the second beam of radiation and generates a third beam of radiation and a detector system that receives the third beam of radiation.
A second aspect of the present invention regards a grating that includes a multilayer structure that has alternating layers of materials, a plurality of grooves formed between a plurality of lands, wherein at least one structural parameter of the plurality of grooves and plurality of lands is formed randomly in the multilayer structure.
An advantage of each aspect of the present invention is to provide crystals that are not sensitive to the environment and do not degrade quickly when used in an analyzing system.
A second advantage of each aspect of the present invention to provide a multilayer grating/mirror that has increased resolution and diffraction efficiency while at the same time contributing in making the detector signal having a large signal to noise ratio.